The increasing service life of telecommunications satellites and the change in the requirements linked to the various missions which may be entrusted to them make it necessary for the payloads, and particularly the antennae, of future generations of satellites to be flexible. This flexibility may be implemented in terms of the geographical coverage zone of the antenna, and/or in terms of the polarization and/or the operating frequency band. This flexibility is not intended to cover all the geographical coverage zones simultaneously, but, instead, to have a choice between a plurality of geographical coverages capable of being generated by the same antenna and to make it possible to modify the satellite's mission in orbit.
The antennae placed on board satellites typically comprise a geometrically shaped reflector illuminated by a single source in order to cover a coverage zone aimed at the Earth. A satellite generally comprises a transmission and reception antenna or a transmission antenna and a reception antenna per coverage zone. The geometrical shape of the reflector may, where appropriate, be defined so as to be optimized for a plurality of orbital positions of the satellite, but, in general, so as to cover a single geographical coverage.
Frequency flexibility over a broad-band spectrum, for example the frequency plane Ku, Ku+ covering the frequencies between 10.7 GHz and 18.4 GHz and a single coverage zone, cannot be obtained by means of a single source, since, at the present time, no source has a sufficiently broad band. Furthermore, there is a critical point regarding the diplexing between the transmission and reception bands, and it is necessary to preserve an allowance of the order of 250 MHz between the high frequency of the transmission band and the low frequency of the reception band.
A first known solution is to use two separate antennae in order to cover the same geographical zone, but this solution presents problems of mass, of bulk and of cost.
A second known solution involves placing two sources side by side in front of an over-dimensioned reflector, so as to minimize the defocusing of the two sources. The phase centers of the two sources are located in the focal plane of the reflector, and their radiation axes are parallel. The two sources are positioned as near as possible to the focal point of the reflector in order to reduce the defocusing of the sources and the directivity losses of the antenna which arise as a result. However, this solution is not optimal.
As an example, one reference discloses an antenna device comprising two sources and a pivotable auxiliary reflector provided with two reflecting surfaces. On the one hand, this device has the abovementioned defocusing disadvantages, thus impairing the performances of the antenna, and, on the other hand, the number of degrees of freedom accessible on an auxiliary reflector is relatively low, which amounts to limiting the deformation possibilities of the coverage obtained by means of the antenna beam.
Another possibility involves using a single source located at the focal point of a reflector, the source being connected to a complex electrical architecture combining two radiofrequency chains, the first chain operating in a first frequency plane and the second chain operating in a second frequency plane. However, this architecture entails a complexity which gives rise to appreciable ohmic losses and a high implementation cost.
Moreover, in order to produce two separate coverage zones, the present solutions make it necessary to use two separate and independent antennae, each comprising a deployable reflector, and the reflector has to be linked to two different sources in order to cover a selected frequency band completely, thus making it necessary to have a total of four sources placed on a side face of a satellite, and a double stacking system for deploying or stowing the two reflectors of the two antennae.
Another reference describes another solution involving using a reversible reflector comprising two reflecting surfaces covering two different coverage zones, the reflector being linked to a single source. Positioning one of the reflecting surfaces in front of the source makes it possible to select one of the coverage zones, but this solution does not have any frequency flexibility and does not make it possible to operate in a broad-band frequency plane.